Amplifier or booster modules are required after respective transmission sections for transmitting an optical WDM signal along long fiber paths. One effective method for additional amplification of a signal is based on stimulated Raman scatter, in which a pump signal is fed into the transmission fiber. The pump signal is produced via a number of pump sources, generally laser diodes. The use of a number of pump wavelengths leads to a broad, flat gain spectrum.
The document EP 0139081 B1, xe2x80x9cOptical Communications System Using Raman Repeaters and Components Thereforxe2x80x9d, discloses a cascaded Raman amplifier system with different amplifiers (RA1, RA2 . . . ). These amplifiers have a number of pump sources (column 8, lines 21-28) which inject their pump power for Raman amplification of a WDM signal into the transmission fiber. The Raman gain spectrum thus becomes flatter; that is, the WDM channels will have similar levels at the output of the transmission fiber that forms the Raman amplifier (column 8, lines 32-35).
The wavelength set of pump sources is chosen such that all the channels of the WDM signal have as uniform a gain as possible, taking account of the Raman gain spectrum (see xe2x80x9cFiber Optic Communication Systemsxe2x80x9d, G. P. Agrawal, 2nd Edition, p. 381, FIG. 8.11). A channel with a frequency shift of 13.2 THz with respect to a pump frequency is amplified to the maximum extent. If there is a smaller or greater frequency difference between a channel and a pump signal, then the channel is amplified to a lesser extent. The use of a greater number of different pump wavelengths allows all the channels in the WDM transmission signal to be amplified more homogeneously.
The use of a large number of pump sources is described, for example, in xe2x80x9c1-THz-Spaced Multi-Wavelength Pumping For Broadband Raman Amplifiersxe2x80x9d, ECOC 2000, 26th European Conference on Optical Communication, Sept. 3-7, Proceedings Vol. 2, Sep. 5, 2000, p. 73-74xe2x80x9d. The pump source has 16 wavelength-division multiplexed pump lasers (laser diodes) with a frequency separation of 1 THz (introduction, xc2xa72, lines 22-23), whose signals are fed into the transmission fiber. The channels of the WDM signal in C-band and L-band (1527-1607 nm) are thus amplified with Raman gain differences of less than 0.1 dB. No additional filter (gain flattening filter) is required. According to FIG. 3 in this reference, a slight ripple is still evident in the gain spectrum, but this is less than 0.1 dB.
In addition, trials also have been carried out with more than 16 pump lasers in order to flatten the gain spectrum. xe2x80x9cMessungen an Erbium-dotierten Faserverstxc3xa4rken fxc3xcr WDM-Systemexe2x80x9d, [Measurements on erbium-doped fiber amplifiers for WDM systems] NTZ (Informationstechnik+Telekommunikation), Issue May 5, 2000, p. 60-63, describes an arrangement with up to 38 lasers in the pump source (FIG. 3, xe2x80x9cInfluence of the number of laser sources on the measurement accuracy for indirect measurementxe2x80x9d). The gain spectrum has a very flat region for a bandwidth of less than 40 nm.
Restricting the number of pump sources for an amplifier causes greater ripple in the amplitude spectrum of the amplified WDM signal. This effect will have an even greater adverse effect on the transmission if a number of identical Raman amplifiers are arranged in cascade for further transmission paths, since these disturbance effects are additive.
What are referred to as gain flattening filters (xe2x80x9cFiber Optic Communication Systemsxe2x80x9d, G. P. Agrawal, 2nd Edition, page 415) allows these channel level differences after each amplifier module to be reduced. These filters have a corresponding greater attenuation for wavelengths at higher levels.
The described methods thus propose ways in which the gain spectrum of a Raman amplifier becomes flatter within the bandwidth (C+L-band or narrower) of the WDM signal to be amplified, by increasing the number of pump sources. Nevertheless, filters are still sometimes required in order to minimize the gain spectrum differences. All these methods are highly complex.
An object of the present invention is, thus, to achieve a flat gain spectrum, with less complexity, in a WDM transmission system with cascaded Raman amplifiers.
In the transmission system according to the present invention, the cascaded Raman amplifiers are provided by pump signals with different wavelength sets, so that the disturbing gain fluctuations along the transmission path are compensated for.
After one Raman amplifier stage, the gain spectrum will still have a relatively large amount of ripple (peaks or troughs). The pump wavelengths of two cascaded Raman amplifiers are therefore chosen such that the gain spectrum of the second amplifier has peaks at the points at which the first amplifier has troughs, and vice versa. The fluctuations of the two gain spectra thus compensate for one another, and the sum of the two gain spectra leads to less ripple. This can be achieved, for example, in the case of two successive amplifier stages by offsetting the pump wavelength sets by about half the pump wavelength interval. The pump wavelength interval in this case refers, by way of example, to the mean interval between the pump wavelengths of spectrally adjacent laser diodes in a pump source. In a transmission system with N cascaded Raman amplifiers, an offset of about 1/N of the pump wavelength interval is chosen between the respective pump wavelength sets. If the gain spectrum does not have periodic artifacts after one or more amplifier stages, that is to say irregularly distributed peaks or troughs, the pump wavelength is corrected individually, so as to provide appropriate compensation for the disturbances in the final gain spectrum.
A transmission path also may be formed by periodic sequences of two or more successive Raman amplifiers, which have different wavelength sets to one another. In configurations such as these, a maximum number of four Raman amplifiers with offset wavelength sets is sufficient in practice. Although a greater number is also feasible, this involves greater complexity.
The use of different pump wavelength sets has the major advantage that only a small number of pump sources are required for each Raman amplifier, since the gain differences are adequately compensated for along all the sections of the transmission path. The WDM signal at the receiving end of the entire path will have minor level differences if the pump wavelengths of the Raman amplifiers are advantageously shifted with respect to one another.
In consequence, and in addition, no filters (gain flattening filters) are required to minimize the level differences after each amplifier or at the end of the transmission path.
The pump power levels and the spectral widths of the pump channels are chosen so as to avoid the effects of stimulated Brillouin scatter SBS. A normal SBS threshold is around a maximum of 10 mW within the Brillouin bandwidth of 20 MHz.
Depending on the requirements, the pump signals can be fed into the transmission sections either codirectionally or contradirectionally.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.